Polymeric membranes can be used to separate liquid and gaseous streams into their component fractions. Unlike distillation, sublimation, and crystallization, membrane separations operate without heating or cooling. Membranes can be used for the separation of a wide variety of gaseous and aqueous streams, for example air, sea water, waste water, and biological fluids, such as blood. The membranes used to accomplish these separations can be fabricated in various geometries, such as a flat sheet, spiral wound sheet, tubular, and hollow fiber. The membrane shape is usually determined by the nature of the separation that is to be effected. When performing a separation on a viscous liquid mixture, for instance, it can be advantageous to use a membrane in a large diameter tubular configuration in order to maintain fluid velocity and to minimize fouling of the membrane surface. Conversely, when separating fluids with low viscosities, such as gases, the use of membranes in a hollow fiber configuration is more efficient.
The fine hollow fiber geometry is particularly advantageous because it can yield very high surface area-to-volume ratios. Much of this benefit is derived from the fact that a membrane support structure is integral to the hollow fiber, i.e., the membrane is a self-supporting entity. This is in contrast to flat sheet and spiral wound membranes that are typically cast onto a nonwoven fabric, or to tubular membranes that are frequently cast onto a rigid porous backing tube. Thus, a significant portion of the module volume of these membranes is consumed by the membrane support structure.
A tube sheet is a plate, sheet, or bulkhead which is perforated with a pattern of holes designed to accept pipes or tubes. When used to support and isolate hollow fiber filter elements, tube sheets can be fabricated with a liquid resin that subsequently solidifies, often by a chemical curing process. An example of a liquid resin used for casting tube sheets are two part amino-epoxy adhesives.
Two part amino-epoxy adhesives exhibit good dimensional stability and high strength at elevated temperature and in harsh environments, which make them suitable for casting tube sheets. Although amino-epoxy adhesives dominate the filtration market, they exhibit undesirable cure characteristics, including a high peak exotherm temperature. High peak exotherm temperatures can degrade plastic parts, for example hollow fibers, embedded in the adhesive, and can cause hollow fiber embrittlement, which reduces filtration efficacy. Amino-epoxy adhesives are also prone to darken.
Two-part polyurethane adhesives are promising replacements for amino-epoxy adhesives for hollow fiber filtration modules. They exhibit good adhesion to hollow fibers and maintain a fluid-tight relationship between the hollow fibers and the tube sheet, thus preventing unwanted components of the feed stream from mixing with the permeate. However, they are prone to high exotherm temperatures, discoloration, and bubble formation. Attempts to develop two-part polyurethane adhesives having a reduced peak exotherm temperature can result in tube sheets having reduced heat distortion temperature, tensile strength, Shore hardness, flexibility, or chemical resistance. For example, castor oil-based polyurethanes can exhibit poor chemical resistance. Higher chemical resistance can be achieved by using polybutadiene polyol. However the cost of this polyol is prohibitive. Moreover, both the castor oil- and polybutadiene polyol-based polyurethanes can have lower heat distortion temperatures.
In order to minimize adverse effects on hollow fibers, multi-part polyurethane adhesives should have reduced peak exotherm temperatures. In view of the deficiencies of castor oil- and polybutadiene polyol-based polyurethanes, any reduction in the peak exotherm temperature should not adversely affect heat distortion temperature, tensile strength, Shore hardness, flexibility, or caustic resistance of the tube sheet. Also desirable are the absence of bubbles, reduced discoloration, and low viscosity for facile mixing and casting.